In electrophotography, an electrophotographic imaging member (i.e., a photoreceptor) is comprised of a stack of typically three or more coatings on a substrate of plastic, or metal. The configuration typically comprises a conductive layer (if the substrate is not metal and/or otherwise an inherently conductive material) on the substrate; a semi conductive and/or charge blocking layer; a generator layer of a photoconductive substance such as selenium and/or selenium alloys, pigments, ZnO, sulfur compounds and others either coated neat or in a polymeric binder; and a polymeric transport layer containing a hole or electron conductive compound that is soluble in the dried polymer coating, i.e. it is a clear homogeneous coating with no apparent crystals of the conductive compound. The device is charged with a high voltage corona, exposed to light reflected off of a document either through a lens or to a laser scanning apparatus that dissipates the charge in the white or background areas to form a positive latent mirror image of the document on the surface of the imaging member. The latent image is then developed with a marking material or toner particles in the approximate size range of 8 to 10 microns that have an opposite charge and are therefore attracted to the latent image. The resulting visible image is transferred from the device to a support such as paper or plastic. This imaging process takes place in seconds or fractions of a second and may be repeated thousands or even hundreds of thousands of times for the life of the device.
An electrophotographic imaging member may have a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or may be a composite layer containing a photoconductor and another material. One type of composite imaging material comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of generating positive holes when exposed to light and injecting them into the transport layer that relieves the electrons and/or net negative charge on the surface.
Other composite imaging members have been developed having numerous layers which are highly flexible and exhibit predictable electrical characteristics within narrow operating limits to provide excellent images over thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a positive hole blocking layer, an adhesive layer, a charge generating layer, and a transport layer. This type of photoreceptor may also comprise additional layers such as an anti-curl back coating and an overcoating layer. It may also require additional adhesive layers or as is the case of the examples in this application require no adhesive layers.
A common type of photoreceptor has a suitable charge generating (photogenerating) layer applied to a charge blocking layer and/or an adhesive layer in between if there is poor adhesion of the photogenerator to the charge blocking layer. Additional layers of adhesives are not the most ideal configuration since they mean another step in manufacturing and they do add indiscriminate charge insulation, thus reducing the overall efficiency of the photoreceptor; however, in many cases such layers are required for the structural integrity of the stack. Examples of photogenerating layers include inorganic photoconductive materials such as amorphous selenium, trigonal selenium, selenium alloys selected from a group consisting of selenium-tellurium, selenium-tellurium-arsenic and selenium arsenide, phthalocyanine pigments such as the X-form of metal free phthalocyanine described in U.S. Pat. No. 3,356,989, metal phthalocyanines such as vanadyl phthalocyanine and copper phthalocyanine, dibromoanthanthrone, squarylium, quinacridones [available from Du Pont under the tradename Monastral Red, Monastral Violet, and Monastral Red Y, and Vat orange 1, and Vat orange 3 (tradenames for dibromo anthanthrone pigments)], benzimidazole perylene, substituted 2,4-diamino-triazines as disclosed in U.S. Pat. No. 3,442,781, and polynuclear aromatic quinones (available from Allied Chemical Corporation under the tradename Indofast Double Scarlet, Indofast Violet Lake B, Indofast Brilliant Scarlet and Indofast Orange). The particles are generally dispersed into a film forming polymeric binder dissolved in a suitable solvent and/or mixture of solvents. Making a good dispersion is in itself no trivial matter. The common methods of mechanical milling do not always afford good particle size and/or distribution in the submicron range. The method of milling employed may create deleterious properties such as increased dark decay increasing as a function of milling time; flocculation of the pigment particles due to solvent and/or polymer; a fast settling dispersion that is difficult to coat before it separates even under agitation; or pigment that is partially soluble in the solvent resulting in recrystallization which causes unacceptable diverse particle size distribution, etc.
Multi-photogenerating layer compositions may be utilized where a layer enhances or reduces the properties of the photogenerating layer. Examples of multiphotogenerating layer image members are described in U.S. Pat. No. 4,415,639. Other suitable photogenerating materials known in the art may also be utilized. Charge generating layers comprising a photoconductive material such as vanadyl phthalocyanine, metal free phthalocyanine, benzimidazole perylene (BZP), amorphous selenium, trigonal selenium, selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide, and mixtures thereof, are especially preferred for their sensitivity to white light. Vanadyl phthalocyanine, metal free phthalocyanine and tellurium alloys are also preferred because these materials are also sensitive to infrared light.
Any suitable polymeric film forming binder material may be employed as the matrix in the photogenerating binder layer. Typical polymeric film forming materials include those described in U.S. Pat. No. 3,121,006.
The photogenerating composition or pigment is present in the resinous binder composition in various amounts. Generally from about 5% by volume to about 90% by volume of the photogenerating pigment is dispersed in about 10% by volume to about 95% by volume of the resinous binder. Preferably, from about 20% by volume to about 30% by volume of the photogenerating pigment is dispersed in about 70% by volume to about 80% by volume of the resinous binder composition.
The photogenerating layer generally ranges in thickness from about 0.1 micrometer to about 5.0 micrometers, preferably from about 0.3 micrometers to about 3.0 micrometers. The photogenerating layer thickness is related to the binder contents. Higher binder content compositions generally require thicker layers. Thicknesses outside these ranges can be selected if layers of greater thickness achieve the objectives of the present invention. The binders main purpose is a mechanical one--to hold the photogenerator substance together within the desired configuration of the total stack of layers of the photoreceptor. Since binders in general are electrical insulators, higher binder content photogenerator layers result in less spectral sensitivity that makes it necessary for stronger or more powerful light exposure and erase lamps, i.e. they become less light sensitive. At low binder contents some photogenerators become too conductive, that is it becomes impossible to hold a charge on the photoreceptor long enough even without exposure to light to make a useful photoreceptor, this type of discharge is called excessive dark decay. Since a modern copy machine cycle is measured seconds, some dark decay can be tolerated. It may be possible in some designs to reduce some dark decay by increasing the binder content.
Current generator coating formulae are usually dispersions of selenium or other photoconductors as described above in solutions of polymers. When the binder/generator layer (BGL) is coated, the solvents, which serve no further purpose once the coating is formed, must be removed by heating. The removal of solvents can result in increased air pollution. It is thus desirable to alternatively make coatings for BGLs by a process that either does not use any solvents or one that greatly reduces their use and as a result does not require solvent removal or only a minimal amount.